Nest 3Gen detailed operation with OpenTherm

[Mike21]

Before I take the plunge into OpenTherm heating control with the Nest 3Gen thermostat, I've read that it doesn't implement the "OpenTherm Low Load" message, so when temperature is approaching/at the desired temperature & the boiler load is low how does the boiler know to switch itself (& the pump) off? Otherwise I'm guessing the pump & boiler will be running 22/7/365? 

I *was* going to ask whether the OT control only applies when the schedule is calling for CH or HW period, but I gather the schedule is actually always running...

So, do the HW and CH switched outputs on the HeatLink still operate with OpenTherm boiler control?

[AlexD]

Thank you for your interest in our products @Mike21 . I would be happy to try and answer as much as I can. 

To start with you can have a look at this link here. It has most of our information about how our thermostat interacts with OpenTherm systems. 

[Mike21]

Thanks for the link, but I have already seen it and, I think, all the other links relating to OpenTherm with Nest. I don't find them very helpful. 

Perhaps I should say I'm a retired systems engineer and would prefer to see technical manuals rather than the user-level.

Someone on a Youtube video has confirmed they have Nest operating on OpenTherm and their boiler does indeed switch off when the room is at the desired temperature, so I guess the thermostat comparison I saw that said Nest doesn't implement OTLL was wrong.

But I still need to know if the HW and CH switches operate according to the schedule when using OT, in order to figure out my zone valve connections.

[AlexD]

I see you're point. There is no other technical documentation I have access that I could share apart from the publicly available ones. 

The heating and hot water will be operate based on the set schedule, if a separate hot water tank compatible with OpenTherm is at the location. You will still set a schedule for the heating and hot water just like when it operates as an on/off switch. You will also have some additional options compared to a standard on /off control such as setting the temperature of the water, you can see more information about this here. 

The Heat Link will be wired using the OT1 and OT2 terminals for OpenTherm control to the boiler. 

[Mike21]

Thanks. If the switches still operate according to the schedule then they can be used to control the S plan valves as per an on/off scheme but with OT providing boiler modulation for thermal efficiency and reduced CO2 emissions. I'll have to get a Cylinder Demand Box to tell the boiler to provide full output temperature for HW cylinder. Someone else was asking about Nest with OT and HW; the answer is the HW switch needs to tell the boiler to output at full temperature- some boilers have two separate switched live feed inputs & one can be used to order this high output. Apparently some boilers have a switched live which overrides the OT control for this purpose. My boiler doesn't do either of those options & I need to get a Cylinder Demand Box to tell it to provide high output using OT control.

Can you confirm the Nest will send the OT boiler disable message when the heating load requirement is less than the minimum boiler modulation, and will enable the boiler again when further heat is required by the Nest?

[CristianC]

Hey @Mike21. The thermostat will turn on your boiler until the set temperature is reached. Once you have the correct temperature in your room it will send a signal to turn it off. When the temperature drops, it will turn on your boiler again. 

[Mike21]

@CristianC 

thank you! That's what I expected to happen but it's nice to get it confirmed! I then have one final question - how does Nest with OpentTherm connection avoid causing boiler cycling when almost at the required set temperature and the heating demand is at low load? Is it purely determined by the thermal hysteresis of the sensor, or are further control measures applied?

[CristianC]

That's a good question. The thermostat has a feature called True Radiant that works well with OT boilers. It will learn how long it takes for your home to heat up and will turn your boiler on/off accordingly. It can even turn it on/off before the scheduled time or before it reaches the set temperature. As an example, it will turn off your boiler earlier because it knows that the radiators are hot and the temperature will be reached by the radiant heat. Or it will turn it on earlier so you already have the desired temperature by the time you scheduled it. You're also right about the thermal hysteresis of the sensor. 

[george_t]

Hi Mike21. Is there anything else we can assist you with at the moment?

[AlexD]

Hey there, I haven't heard from you in a while so I'll be locking this thread. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the community. Thanks, AlexD.